(a) Field of the Invention
The present invention relates to a liquid crystal display and, more particularly, to a twisted nematic (TN) mode liquid crystal display.
(b) Description of the Related Art
Generally, a liquid crystal display has a structures where a liquid crystal layer is sandwiched between two substrates, and an electric field is applied to the liquid crystal to control light transmission. Of the two substrates, the bottom substrate is provided with thin film transistors and pixel electrode, and the top substrate with a common electrode and color filters.
The twisted nematic (TN) mode has been mainly employed for use in a large size and high definition liquid crystal display because it has the advantages of structural stability and simplified processing steps. In the TN mode liquid crystal displays, the substrates are rubbed for alignment such that the directors of the liquid crystal molecules at the top substrate are perpendicular to those of the liquid crystal molecules at the bottom substrate.
In order to enhance the viewing angle, a multi-domain technique has been developed for the TN mode liquid crystal displays. In the multi-domain liquid crystal display, a number of differently-structured liquid crystal domains are present at one pixel area. Assuming that a group of liquid crystal molecules with the same direction of twisting is referred to as the xe2x80x9cdomain,xe2x80x9d the multi-domain liquid crystal display bears multiple groups of liquid crystal molecules at one pixel area.
FIG. 1 illustrates a sectional structure of a two-domain twisted nematic (TDTN) liquid crystal display at one pixel area according to a prior art.
As shown in FIG. 1, the liquid crystal panel 210 includes a bottom substrate 201, a top substrate 202, and a liquid crystal layer 209 sandwiched between the bottom and the top substrates 201 and 202. A first liquid crystal domain A where the liquid crystal molecules are twisted in a first direction 1 is placed at the left side L of the pixel. A second liquid crystal domain B where the liquid crystal molecules are twisted in a second direction 2 is placed at the right side R of the pixel.
The two liquid crystal domains A and B may be formed through differentiating the pretilt angles of the liquid crystal molecules at the bottom substrate 201 or the top substrate 202.
For Instance, the liquid crystal molecules placed at a predetermined region of the bottom substrate are established to have a large pretilt angle, whereas those at the corresponding region of the top substrate to have a small pretilt angle. Furthermore, the liquid crystal molecules placed at another region of the bottom substrate are established to have a small pretilt angle, whereas those at the corresponding region of the top substrate to have a high pretilt angle. Even though the liquid crystal molecules placed close to the substrates are oriented depending upon the respective pretilt angles due to the condition of the substrate, the liquid crystal molecules within the liquid crystal layer are oriented pursuant to the higher pretilt angle, resulting in two or more liquid crystal domains.
In the drawing, the respective liquid crystal domains A and B bear different pretilt angles with respect to the bottom and the top substrates 201 and 202. For instance, the liquid crystal molecules at the first liquid crystal domain A are tilted against the bottom substrate 201 at an angle of about 6-7xc2x0 while being tilted against the top substrate 202 at an angle of about 0-1xc2x0. By contrast, the liquid crystal molecules at the second liquid crystal domain B are tilted against the bottom substrate 201 at an angle of about 0-1xc2x0 while being tilted against the top substrate 202 at an angle of about 6-7xc2x0. The inclined lines at the bottom and the top substrates 201 and 202 indicate the pretilt angles of the liquid crystal molecules.
The pretilt angles of the liquid crystal molecules are determined depending upon the surface roughness of alignment films (not shown). The surface roughness of the alignment film varies depending upon the rubbing conditions, the amount of light exposure, and the surface roughness of the ITO-based layer. Conventionally, the ITO-based pixel electrode of the bottom substrate 201 and the ITO-based common electrode of the top substrate 202 have various surface roughness, thereby controlling the pretilt angles of the liquid crystal molecules close to the respective substrates. When the surface roughness of the pixel electrode is high, the pretilt angle of the liquid crystal molecules is reduced, whereas when the surface roughness of the pixel electrode is low, the pretilt angle of the liquid crystal molecules increases. Therefore, the pretilt angles of the liquid crystal molecules can be controlled based on the surface roughness of the pixel electrode.
In order to form such a pixel electrode, after the deposition of the pixel electrode layer, a photoresist pattern is formed on the pixel electrode layer while exposing the portion to be surface-treated, and the exposed portion of the pixel electrode layer is wet-etched using the photoresist pattern as a mask.
However, in the above technique, a separate mask should be provided to make surface treatment in addition to form the pixel electrode layer. This complicates the processing steps and lowers production efficiency.
It is an object of the present invention to provide a method for fabricating a multi-domain liquid crystal display bearing wide viewing angle characteristics without using an additional mask.
This and other objects may be achieved by a method for fabricating a liquid crystal display where a pixel electrode with different surface roughness is formed at each pixel area using one mask.
According to one aspect of the present invention, in a method of fabricating a liquid crystal display, a pixel electrode is formed on a bottom substrate at each pixel area using a first mask such that the pixel electrode has a first region with a smooth surface, and a second region with a rough surface, the bottom substrate having a gate wire, a data wire and a thin film transistor. And, a common electrode is formed on a top substrate using a second mask such that the common electrode has a first region with a smooth surface, and a second region with a rough surface, the top substrate having color filters; and the bottom substrate having the pixel electrode. And the top substrate is assembled with the bottom substrate, and liquid crystal is injected between the bottom substrate and the top substrate.
In order to form the bottom substrate a gate wire is formed on a first insulating substrate. The gate wire includes gate line, and gate electrode. A gate insulating layer is formed on the substrate such that the gate insulating layer covers the gate wire. A semiconductor pattern and a data wire are formed on the gate insulating layer. The data wire includes data line, source electrode connected to the data line while being connected to the semiconductor patter, and drain electrode facing the source electrode while being connected to the semiconductor pattern. A protective layer is formed on the substrate such that the protective layer covers the data wire. First contact holes are formed at the protective layer such that the first contact holes expose the drain electrode.
The first mask, has a first region transmitting light with, a first light transmissivity, and a second region transmitting the light with a second light transmissivity lower than the first transmissivity. The first and the second regions of the first mask define the shape of the pixel electrode. The first region has a semitransparent pattern, and the second region has an opaque pattern. Each of the first region and the second region consists of a plurality of sub-regions, and the sub-regions of the first region and the second region are alternately arranged.
In order to form the pixel electrode, a transparent conductive layer is deposited over the bottom substrate. A photoresist film is coated on the transparent conductive layer. The photoresist film is selectively exposed to light using the mask. A photoresist pattern is formed on the conductive layer. The photoresist pattern has a first photoresist portion placed over the first region of the pixel electrode with a first thickness, and a second photoresist portion placed over the second region of the pixel electrode with a second thickness larger than the first thickness. The transparent conductive layer is etched using the photoresist pattern as a mask. The first photoresist portion is removed while exposing the underlying transparent conductive layer. The first region of the pixel electrode is formed through surface-treating the exposed portion of the transparent conductive layer. The second photoresist portion is removed while exposing the second region of the pixel electrode.
The surface may be treated through bombarding inert gas on the exposed portion of the transparent conductive layer, or through wet-etching the exposed portion of the transparent conductive layer using a wet etching solution. The first and the second portions of the photoresist pattern may be removed through dry etching.
The semiconductor pattern and the data wire are formed, through photolithography using a photoresist pattern having different thickness. The photoresist pattern has a first photoresist portion placed over the data wire with a first thickness, and a second photoresist portion, placed over the source and the drain electrode with a second thickness smaller than the first thickness.
In order to form the semiconductor pattern and the data wire, a semiconductor layer and a conductor layer are deposited on the gate insulating layer, and the photoresist pattern is formed on the conductive layer. The conductive layer is etched using the photoresist pattern as a mask while partially exposing the semiconductor layer. A semiconductor pattern is completed through removing the exposed portion of the semiconductor layer and the second portion of the photoresist pattern while partially exposing the conductive layer between the source and the drain electrode. A data wire is formed through removing the exposed portion of the conductive layer, and the first portion of the photoresist pattern is removed. The photoresist pattern may be formed using a mask having a first region with a predetermined light transmissivity, a second region with a light transmissivity lower than the light transmissivity of the first region, and a third region with a light transmissivity higher than the light transmissivity of the first region.
The step of forming the data wire may be made after the semiconductor pattern is formed on the gate insulating layer.
In order to form the top substrate, color filters are formed on a second insulating substrate A common electrode is formed on the substrate such that the common electrode covers the color filters. The common electrode may be formed in the following way. A transparent conductive layer is deposited over the second insulating substrate such that the transparent conductive layer covers the color filters. A photoresist film is coated on the transparent conductive layer. The photoresist film is selectively exposed to light using the second mask. A photoresist pattern is formed on the transparent conductive layer through developing the light-exposed photoresist film. The photoresist pattern has a first portion placed over the first region of the common electrode with a first thickness, and a second portion placed over the second region of the common electrode with a second thickness larger than the first thickness. The transparent conductive layer is etched using the photoresist pattern as a mask. The first portion of the photoresist pattern is removed while exposing the underlying transparent conductive layer. The first region of the common electrode is formed through surface-treating the exposed portion of the transparent conductive layer, and the second region of the common electrode is formed through removing the second portion of the photoresist pattern. The surface may be treated through bombarding inert gas on the exposed portion of the transparent conductive layer, or through wet-etching the exposed portion of the transparent conductive layer using a wet etching solution.
The first and the second portions of the photoresist pattern may be removed through dry etching.
The resulting liquid crystal display includes a bottom substrate with a first region where liquid crystal molecules bear a first pretilt angle, and a second region where liquid crystal molecules bear a second pretilt angle larger than the first pretilt angle. A top substrate faces the bottom substrate with liquid crystal molecules bearing a third pretilt angle. The third pretilt angle mediates between the first and the second pretilt angles. A liquid crystal layer is sandwiched between the bottom and the top substrates with liquid crystal molecules. The liquid crystal molecules of the liquid crystal layer are twisted at the first region in a first direction while being twisted at the second region in a second direction.
The bottom substrate includes a gate wire, and a data wire crossing over the gate wire while being insulated from the gate wire. Thin film transistors are electrically connected to the gate wire and the data wire, and pixel electrode are electrically connected to the thin film transistors. Each pixel electrode bears a first surface roughness at the first region while bearing a second surface roughness at the second region. The second surface roughness is higher than the first surface roughness.
The top substrate includes a common electrode corresponding to the pixel electrode with a third surface roughness medium between the first surface roughness and the second surface roughness. An alignment film on the common electrode may have grooves such that the liquid crystal molecules close thereto bear a third pretilt angle.
According to another aspect of the present invention, in a method for fabricating a liquid crystal display, a bottom substrate is formed such that it has a first region where liquid crystal molecules bear a first pretilt angle, and a second region where liquid crystal molecules bear a second pretilt angle smaller than the first pretilt angle. A top substrate is formed such that it faces the bottom substrate with liquid crystal molecules bearing a third pretilt angle the third pretilt angle medium between the first pretilt angle and the second pretilt angle. A liquid crystal layer is formed between the bottom and the top substrates with liquid crystal molecules. The liquid crystal molecules of the liquid crystal layer are twisted at the first region in a first direction while being twisted at the second region in a second direction.
In order to form the bottom substrate, a gate wire, a data wire and thin film transistor are formed on a first insulating substrate such that the data wire crosses over the gate wire while being insulated from the gate wire, and the thin film transistors are electrically connected to the data wire. Pixel electrode is formed such that they are electrically connected to the thin film transistors. Each pixel electrode bears a first surface roughness at the first region while bearing a second surface roughness at the second region. The second surface roughness is higher than the first surface roughness.
In order to form the pixel electrode, a transparent conductive layer is deposited over the top substrate. A photoresist film is coated on the transparent conductive layer. The photoresist film is selectively exposed to light using a mask. A photoresist pattern is formed on the transparent conductive layer through developing the light-exposed photoresist film. The photoresist pattern has a first portion placed over the first region of the common electrode with a first thickness, and a second portion placed over the second region of the common electrode with a second thickness larger than the first thickness. The transparent conductive layer is etched using the photoresist pattern as a mask. The first portion of the photoresist pattern is removed while exposing the underlying transparent conductive layer. The first region of the pixel electrode is formed through surface-treating the exposed portion of the transparent conductive layer. The second region of the pixel electrode is formed through removing the second portion of the photoresist pattern. The surface may be treated through bombarding inert gas on the exposed portion of the transparent conductive layer, or through wet-etching the exposed portion of the transparent conductive layer using a wet etching solution.
The top substrate may be formed through forming a common electrode on a second insulating substrate such that it faces the pixel electrode with a third surface roughness medium between the first surface roughness and the second surface roughness. The common electrode maybe formed through depositing a transparent conductive layer over the top substrate, and surface-treating the transparent conductive layer such that the transparent conductive layer bears the third surface roughness. The surface may be treated through bombarding inert gas on the exposed portion of the transparent conductive layer, or through wet-etching the exposed portion of the transparent conductive layer using a wet etching solution.
Furthermore, the top substrate may be formed in the following way. A common electrode is formed on a second insulating substrate such that the common electrode faces the pixel electrode. An alignment film is coated over the substrate such that the alignment film covers the common electrode. The alignment film is rubbed such that the liquid crystal molecules at the top substrate bear the third pretilt angle.